Study On The Extracting Conditions Of Silicon And Aluminum From Fly Ash

As a kind of industrial solid waste,fly ash not only occupies land but also causes serious pollution to the environment.At the same time,fly ash is a very important resource of aluminum and silicon,which is of great utilized value as a renewable resource.In this dissertation,calcinated activation-acid leaching process route were adopted to treat fly ash,basing on the previous laboratory research results and literature research.The screening of activator,the process conditions of the calcination activation and the preparation of gamma-Al₂O₃ and nano-silica products were studied.The main research work and conclusions were as follows:?1?CaCl₂+Ca?OH?₂ was selected as the more suitable compound activator for the calcination of fly ash;the calcination activation conditions were optimized as follows:mass ratio of fly ash to CaCl₂ to Ca?OH?₂=2:1:0.2,calcination time 1.5h,calcination temperature 800?;the acid leaching conditions were optimized as follows:reaction temperature 95?,reaction time 3.5h,the concentration of H₂SO₄ 33%?W_t%?,the ratio of liquid to solid 3:1.Under the above conditions,the average leaching rate of Al₂O₃could reach 95.5%while the average ratio of the residual acid in the leachate was only4.9%.?2?Gamma-Al₂O₃ were prepared by precipitation and refining of crude aluminium ammonium sulfate?AAS?,precipitation and calcination of refined AAS from acid leaching filtrate and then the more suitable process conditions were optimized.Preparation of crude AAS:molar ratio of NH₄⁺/Al³⁺1.3:1,reaction time 4.5 h,aging time 0.5 h,under this condition,the average yield of Al³⁺in acid leachate could reach80.73%;recrystallization of crude AAS:oxalic acid was selected as the more suitable iron remover,the addition amount in two recrystallization was 2%and 1%respectively;preparation of aluminium hydroxide from refined ammonium alum:the concentration of Al³⁺0.7 mol·L^-1,the terminal pH value 8,reaction temperature 70?and aging time3 h,under this condition,the average yield of Al³⁺could reach 98.9%.The preparation of gamma-Al₂O₃ by calcination of aluminium hydroxide:calcination temperature800?,calcination time 3 h,Under this condition,the yield of Al³⁺was nearly 100%,the purity of the as-obtained product could reach 98.5%,which performing good dispersion with the particle size of 20-50 nm,and meeting the relevant technical requirements of alumina standard?YS/T 274-1998?.?3?The acid leaching residue of activated fly ash was decalcified with ammonia water and ammonium bicarbonate,and nano-CaCO₃ was prepared using the mixture of decalcification filtrate and the water-washing solution of activated fly ash,which performing good uniformity and dispersion with the particle size of 50 nm.The purity and particle size of the as-obtained product could meet the technical requirements of nano-CaCO₃ standard?GB/T 19590-2011?.?4?The process conditions for the alkali leaching of the acid leaching residue were optimized as follows:the concentration of NaOH 25%,the ratio of liquid to solid 3.5:1,leaching temperature 95?,leaching time 90 min,under which the average leaching rate of SiO₂ could reach 94.65%and the mass fraction of SiO₂ in the alkali leaching leachate was 16.1%with the modulus of 1.03.When diluting the above alkali leaching leachate to 5%,and then nano-silica was prepared by carbonation of CO₂ which performing good uniformity and dispersion with particle size of 10-30 nm.